Catalyst for purifying exhaust gases and process for producing the same

ABSTRACT

With respect to an oxide powder having a characteristic that a suspension suspending the oxide powder in pure water exhibits a pH value of 7 or less, a noble metal is loaded on the oxide powder by using a noble metal salt solution exhibiting a pH value lower than the pH value of the suspension in order to inhibit the granular growth of loaded noble metal particles at high temperatures. It is believed that the affinity enlarges between noble metal particles, generated by the decomposition of the noble metal salt, and the oxide powder because no coarse noble metal particles are generated by neutralizing the noble metal salt so that the binding force enlarges between the oxide powder and the noble metal salt.

TECHNICAL FIELD

The present invention relates to a catalyst for purifying exhaust gasesof internal combustion engines for automobiles and the like, and aprocess for producing the same.

BACKGROUND ART

A catalyst (three-way catalyst) for purifying exhaust gases comprises,for example, a support substrate composed of heat-resistant ceramics,such as cordierite, a catalyst loading layer formed on this supportsubstrate and composed of activated alumina and the like, and a noblemetal, such as Pt, loaded on this catalyst loading layer. This three-waycatalyst purifies hydrocarbons (HC) and carbon monoxide (CO) byoxidation, and purifies nitrogen oxides (NO_(x)) by reduction.

However, since the oxygen concentration in exhaust gases fluctuatesgreatly depending on running conditions and the like, there might ariseinstances where the purifying activity of oxidation and reductionbecomes unstable in three-way catalysts. Hence, it has been carried outadding CeO₂ to the catalyst loading layer. CeO₂ has an oxygenstorage-and-release ability (hereinafter referred to as “OSC”) by whichit stores oxygen in oxidizing atmospheres and releases oxygen inreducing atmospheres, and thereby it is possible to obtain a stablepurifying activity even when the oxygen concentration in exhaust gasesfluctuates.

Moreover, three-way catalysts including CeO₂ are such that the OSC islikely to be lowered by the crystalline growth of CeO₂ and the granulargrowth of noble metal accompanied therewith when they are used at hightemperatures of 800° C. or more. Accordingly, in order to maintain ahigh OSC by inhibiting the crystalline growth of CeO₂, it has beencarried out using CeO₂—ZrO₂ system composite oxides.

For example, in Japanese Unexamined Patent Publication (KOKAI) No.2000-176,282, a catalyst is disclosed which comprises a CeO₂—ZrO₂ solidsolution, whose proportion of Ce to Zr is fallen in a specific range, aporous substance such as Al₂O₃, the CeO₂—ZrO₂ solid solution and poroussubstance used as a support, and a noble metal loaded on one of them atleast. In accordance with this catalyst, it is possible to inhibit theOSC from lowering, and the sulfur-poisoning resistance is improved.

Moreover, in Japanese Patent Publication No. 2,659,796, a catalyst isdisclosed which comprises a CeO₂—ZrO₂ system composite oxide, aheat-resistant inorganic oxide, such as Al₂O₃, and a noble metal, andthere is set forth that the durability is improved and high purifyingperformance is revealed.

However, due to the recent improvements of engine performance and beingaccompanied by the increment of high-speed driving, the temperature ofexhaust gases has been increased sharply. Accordingly, the temperatureof catalysts for purifying exhaust gases has rose remarkably as well inservice, compared with that of conventional ones, and consequently ithas become difficult to inhibit the granular growth of noble metal evenwhen the solid solution of CeO₂—ZrO₂ system composite oxides is used.

DISCLOSURE OF INVENTION

The present invention has been done in view of such circumstances, andaccordingly its object is to further inhibit the granular growth ofnoble metal at high temperatures.

A feature of a catalyst according to the present invention for purifyingexhaust gases, catalyst set forth in claim 1 which solves theaforementioned problem, lies in that it comprises: an oxide powderhaving a characteristic that a suspension suspending the oxide powder inpure water exhibits a pH value of 7 or less; and a noble metal loaded onthe oxide powder by using a noble metal salt solution exhibiting a pHvalue lower than the pH value of the suspension suspending the oxidepowder in pure water.

Moreover, a feature of a process according to the present invention forproducing a catalyst for purifying exhaust gases lies in that itcomprises the steps of: preparing an oxide powder having acharacteristic that a suspension suspending the oxide powder in purewater exhibits a pH value of 7 or less; and loading a noble metal on theoxide powder by using a noble metal salt solution exhibiting a pH valuelower than the pH value of the suspension suspending the oxide powder inpure water.

In the present catalyst for purifying exhaust gases and process forproducing the same, it is preferable that the oxide powder can be aCeO₂-based oxide including CeO₂ at least, and it is desirable that theoxide powder can include at least one element selected from the groupconsisting of Zr, La, Y and Nd.

Moreover, it is preferable that the noble metal salt solution can be aPt salt aqueous solution, and it is desirable that a difference (ΔpH)between the pH value of the suspension suspending the oxide powder inpure water and the pH value of the noble metal salt solution can be from1 to 5.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph for illustrating the relationship between the pH valueof suspensions used in examples, suspensions which suspended compositeoxides in pure water, and the CO 50% conversion temperature.

BEST MODE FOR CARRYING OUT THE INVENTION

The mechanism how noble metal particles loaded on supports growgranularly is believed to result mainly from the evaporation andre-precipitation of noble metal particles at high temperatures.Therefore, in order to inhibit the granular growth, it is believed to beeffective to strengthen the electronic interaction between noble metalparticles and supports, or to inhibit the evaporation by modifying andthe like the surface of noble metal particles.

On the other hand, in the conventional methods of loading noble metals,noble metals are loaded on supports in liquid phases by adsorbing noblemetal salts to the supports or impregnating supports with noble metalsalts and thereafter by decomposing the noble metal salts by means ofheat treatments. However, in the methods, since the affinity (chemicalbonding force) is less between noble metal particles, generated by thedecomposition, and supports, it is difficult to inhibit the granulargrowth of noble metal particles at high temperatures.

Hence, in the present process for producing a catalyst for purifyingexhaust gases, with respect to an oxide powder having a characteristicthat a suspension suspending the oxide powder in pure water exhibits apH value of 7 or less, a noble metal salt solution exhibiting a pH valuelower than the pH value of the suspension is used. When an oxide powderwhose suspension exhibits a pH value of 7 or less is used, since noblemetal salts are not neutralized while loading noble metal salts, nocoarse noble metal particles are generated in aqueous solutions. And,when a noble metal salt solution exhibiting a pH value lower than the pHvalue of the suspension is used, it is believed that the affinitybetween noble metal particles, generated by the decomposition of thenoble metal salt, and the oxide powder enlarges.

Therefore, in accordance with the present production process, since finenoble metal particles can be loaded, and additionally the affinitybetween the oxide powder and the noble metal particles is strengthened,it is believed that not only the noble metal particles are inhibitedfrom moving at high temperatures but also the noble metal particles areinhibited from evaporating.

When the pH value of the suspension suspending the oxide powder in purewater exceeds 7, since the noble metal salt is neutralized while loadingthe noble metal salt, coarse noble metal particles are generated inaqueous solutions, and they are loaded on the oxide powder. When suchcoarse particles exist, there arises a problem that not only thecatalytic activity has lowered but also the granular growth at hightemperatures has been further facilitated.

Moreover, when the pH value of the noble metal salt solution is the pHvalue of the suspension or more, the bonding force between the oxidepowder and the noble metal salt is weak. Accordingly, the affinitybetween noble metal particles, generated by decomposing the noble metalsalt, and the oxide powder has been weakened so that the granular growthhas occurred at high temperatures to coarsen and lower the catalyticactivity greatly.

As for the oxide powder having a characteristic that a suspensionsuspending the oxide powder in pure water exhibits a pH value of 7 orless, it is possible to use CeO₂-based oxides, which are produced by aco-precipitation method, for example. In accordance with theco-precipitation method, it is possible to make the pH value of thesuspension 7 or less with ease by controlling the calcination conditions(temperature, time, temperature increment rate and atmosphere) of theprecipitates of generated oxide precursors.

Moreover, when the pH value of the suspension exceeds 7, it is possibleto make the pH value 7 or less by modifying the superficial quality orstate by means of a pretreatment. As for the pretreatment, there is amethod of treating the oxide powder with acids. For example, after theoxide powder is immersed in an acid aqueous solution of nitric acid,acetic acid, hydrochloric acid and the like, it is possible to make thepH value 7 or less by filtering, washing and drying it and followed bycalcining it at 250-500° C. for 2-12 hours. In this instance, as for theacid, those which do not reside after the treatment are preferable, andthose which do not include the S element and the Cl element aredesirable.

In addition, as for the pretreatment, there is a method of exposing theoxide powder to a gas including CO₂. In this instance, the CO₂concentration in the gas can be an equal mol or more to the oxide powderto be treated.

The noble metal salt solution is such that it is possible to use thosewhich exhibit a pH value lower than the pH value of the suspension. Asfor the noble metal, Pt, Rh, Pd, Ir and the like can be exemplified,and, as for the salt, there are ammine nitrates, nitrates,hydrochlorides, acetates, and so forth. The present invention isespecially effective in the case where Pt salt aqueous solutions areused.

Moreover, it is desirable that the difference (ΔpH) between the pH valueof the suspension and the pH value of the noble metal salt solution canbe from 1 to 5. When the ΔpH is fallen in this range, it is possible tofurther inhibit the granular growth of noble metals. For example, whenthe pH value of the noble metal salt solution is from 2 to 3, the pHvalue of the suspension can be adjusted so as to be from 4 to 7. Inaddition, the ΔpH is such that a range of from 1 to 3 is especiallydesirable.

When loading the noble metal on the oxide powder, it can be carried outby impregnating a predetermined amount of the oxide powder with apredetermined amount of the noble metal salt aqueous solution and dryingand calcining it. Moreover, it can be loaded by forming a coating layerof the oxide powder on the surface of honeycomb substrates, impregnatingit with the noble metal salt aqueous solution, and followed by dryingand calcining it.

As for the oxide powder, those whose suspension exhibits a pH value of 7or less can be used, can be selected from Al₂O₃, CeO₂, ZrO₂, CeO₂—ZrO₂,TiO₂ and the like, but can preferably be a CeO₂-based oxide includingCeO₂ at least. This is because CeO₂-based oxides are such that it ispossible to make the pH value of the suspension 7 or less with ease byproducing them by means of the co-precipitation method as set forthabove. Moreover, this is because noble metals loaded on CeO₂ are muchless likely to cause the granular growth compared with the case wherethey are loaded on the other oxides so that it is possible to furtherinhibit the granular growth.

As for the CeO₂-based oxide, it is desirable to include at least oneelement selected from the group consisting of Zr, La, Y and Nd. Whenthese elements are added, it is possible to inhibit the granular growthof CeO₂ at high temperatures, and accordingly it is possible to furtherinhibit the granular growth of the loaded noble metal. Note that theaddition amount of these elements is such that, by molar ratio, Zr candesirably be in a range of Zr/Ce=0.1-10 with respect to Ce; La candesirably be in a range of La/Ce=0.01-5 with respect to Ce; Y candesirably be in a range of Y/Ce=0.01-5 with respect to Ce; and Nd candesirably be in a range of Nd/Ce=0.01-5 with respect to Ce.

Namely, in accordance with the present catalyst for purifying exhaustgases, since it is possible to inhibit the granular growth of loadednoble metals, the durability of purifying activities is improvedgreatly. Moreover, in accordance with the present production process, itis possible to produce the present catalyst for purifying exhaust gaseseasily and securely.

EXAMPLES

Hereinafter, the present invention will be described in detail withreference to examples and comparative examples.

Example No. 1

50 parts by weight of cerium nitrate and 50 parts by weight of zirconiumoxynitrate were solved in pure water to prepare a mixture aqueoussolution, while stirring it, ammonia water was added in an equivalentweight or more for neutralizing the nitrate ions to generateprecipitates. They were washed and filtered, were dried in air at 250°C. for 4 hours, and were thereafter calcined at 700° C. for 2 hours,thereby preparing a CeO₂—ZrO₂ composite oxide powder. When thisCeO₂—ZrO₂ composite oxide powder was suspended in pure water, the pHvalue of the suspension was 6.8.

This CeO₂—ZrO₂ composite oxide powder was impregnated with apredetermined amount of a Pt(NO₂)₂(NH₃)₂ aqueous solution, after dryingand evaporating it, was calcined at 250° C. for 4 hours, therebypreparing a catalyst powder. The pH value of the Pt(NO₂)₂(NH₃)₂ aqueoussolution was 2.2, and the loading amount of Pt was 1.0% by weight.

This catalyst powder was pelletized by an ordinary method, therebymaking a pelletized catalyst.

Example No. 2

Except that 65 parts by weight of cerium nitrate, 30 parts by weight ofzirconium oxynitrate and 5 parts by weight of yttrium nitrate were usedas starting raw materials, a CeO₂—ZrO₂—Y₂O₃ composite oxide powder wasprepared in the same manner as Example No. 1. When this CeO₂—ZrO₂—Y₂O₃composite oxide powder was suspended in pure water, the pH value of thesuspension was 5.7.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the Pt(NO₂)₂(NH₃)₂ aqueous solution was 2.2.

Example No. 3

Except that 60 parts by weight of cerium nitrate, 35 parts by weight ofzirconium oxynitrate and 5 parts by weight of lanthanum nitrate wereused as starting raw materials, a CeO₂—ZrO₂—La₂O₃ composite oxide powderwas prepared in the same manner as Example No. 1. When thisCeO₂—ZrO₂—La₂O₃ composite oxide powder was suspended in pure water, thepH value of the suspension was 5.6.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt(NO₂)₂(NH₃)₂ aqueous solution was2.2.

Example No. 4

Except that 60 parts by weight of cerium nitrate, 35 parts by weight ofzirconium oxynitrate and 5 parts by weight of lanthanum nitrate wereused as starting raw materials, a CeO₂—ZrO₂—La₂O₃ composite oxide powderwas prepared in the same manner as Example No. 1. When thisCeO₂—ZrO₂—La₂O₃ composite oxide powder was suspended in pure water, thepH value of the suspension was 4.8.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt (NO₂)₂(NH₃)₂ aqueous solution was2.2.

Example No. 5

Except that 60 parts by weight of cerium nitrate, 35 parts by weight ofzirconium oxynitrate and 5 parts by weight of lanthanum nitrate wereused as starting raw materials, a CeO₂—ZrO₂—La₂O₃ composite oxide powderwas prepared in the same manner as Example No. 1. When thisCeO₂—ZrO₂—La₂O₃ composite oxide powder was suspended in pure water, thepH value of the suspension was 4.8.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt (NO₂)₂(NH₃)₂ aqueous solution was3.4.

Example No. 6

Except that 50 parts by weight of cerium nitrate, 45 parts by weight ofzirconium oxynitrate and 5 parts by weight of lanthanum nitrate wereused as starting raw materials, a CeO₂—ZrO₂—La₂O₃ composite oxide powderwas prepared in the same manner as Example No. 1. When thisCeO₂—ZrO₂—La₂O₃ composite oxide powder was suspended in pure water, thepH value of the suspension was 6.0.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt (NO₂)₂(NH₃)₂ aqueous solution was2.2.

Example No. 7

Except that 60 parts by weight of cerium nitrate, 35 parts by weight ofzirconium oxynitrate and 5 parts by weight of neodymium nitrate wereused as starting raw materials, a CeO₂—ZrO₂—Nd₂O₃ composite oxide powderwas prepared in the same manner as Example No. 1. When thisCeO₂—ZrO₂—Nd₂O₃ composite oxide powder was suspended in pure water, thepH value of the suspension was 5.9.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt (NO₂)₂(NH₃)₂ aqueous solution was2.2.

Comparative Example No. 1

Except that 65 parts by weight of cerium nitrate and 35 parts by weightof zirconium oxynitrate were used as starting raw materials, and thatthe calcining condition of the precipitates was changed, a CeO₂—ZrO₂composite oxide powder was prepared in the same manner as Example No. 1.When this CeO₂—ZrO₂ composite oxide powder was suspended in pure water,the pH value of the suspension was 8.8.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt (NO₂)₂(NH₃)₂ aqueous solution was2.2.

Comparative Example No. 2

Except that 60 parts by weight of cerium nitrate, 35 parts by weight ofzirconium oxynitrate and 5 parts by weight of lanthanum nitrate wereused as starting raw materials, and that the aging condition of theprecipitates was changed, a CeO₂—ZrO₂—La₂O₃ composite oxide powder wasprepared in the same manner as Example No. 1. When this CeO₂—ZrO₂—La₂O₃composite oxide powder was suspended in pure water, the pH value of thesuspension was 8.2.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt (NO₂)₂(NH₃)₂ aqueous solution was2.2.

Comparative Example No. 3

Except that 60 parts by weight of cerium nitrate, 35 parts by weight ofzirconium oxynitrate and 5 parts by weight of lanthanum nitrate wereused as starting raw materials, and that the aging condition of theprecipitates was changed, a CeO₂—ZrO₂—La₂O₃ composite oxide powder wasprepared in the same manner as Example No. 1. When this CeO₂—ZrO₂—La₂O₃composite oxide powder was suspended in pure water, the pH value of thesuspension was 8.5.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt(NO₂)₂(NH₃)₂ aqueous solution was2.2.

Comparative Example No. 4

Except that 60 parts by weight of cerium nitrate, 35 parts by weight ofzirconium oxynitrate and 5 parts by weight of lanthanum nitrate wereused as starting raw materials, and that the aging condition of theprecipitates was changed, a CeO₂—ZrO₂—La₂O₃ composite oxide powder wasprepared in the same manner as Example No. 1. When this CeO₂—ZrO₂—La₂O₃composite oxide powder was suspended in pure water, the pH value of thesuspension was 8.5.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No.1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt (NO₂)₂(NH₃)₂ aqueous solution was3.4.

Example No. 8

The CeO₂—ZrO₂—La₂O₃ composite oxide powder (the pH value of thesuspension=8.2) prepared in Comparative Example No. 2 was used, wasimmersed in a nitric acid aqueous solution whose pH value=2 for 2 hours.It was filtered and washed, was dried at 250° C. for 4 hours, and wasthereafter calcined at 500° C. for 2 hours. The pH value of a suspensionsuspending the resulting pretreated CeO₂—ZrO₂—La₂O₃ composite oxidepowder was 4.4.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No.1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt(NO₂)₂(NH₃)₂ aqueous solution was2.2.

Example No. 9

The CeO₂—ZrO₂—La₂O₃ composite oxide powder (the pH value of thesuspension=8.2) prepared in Comparative Example No. 2 was used, wasimmersed in an acetic acid aqueous solution whose pH value=2 for 2hours. It was filtered and washed, was dried at 250° C. for 4 hours, andwas thereafter calcined at 500° C. for 2 hours. The pH value of asuspension suspending the resulting pretreated CeO₂—ZrO₂—La₂O₃ compositeoxide powder was 5.3.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt(NO₂)₂(NH₃)₂ aqueous solution was2.2.

Example No. 10

The CeO₂—ZrO₂—La₂O₃ composite oxide powder (the pH value of thesuspension=8.2) prepared in Comparative Example No. 2 was used, wasimmersed in a hydrochloric acid aqueous solution whose pH value=2 for 2hours. It was filtered and washed, was dried at 250° C. for 4 hours, andwas thereafter calcined at 500° C. for 2 hours. The pH value of asuspension suspending the resulting pretreated CeO₂—ZrO₂—La₂O₃ compositeoxide powder was 4.3.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt(NO₂)₂(NH₃)₂ aqueous solution was2.2.

Example No. 11

The CeO₂—ZrO₂—La₂O₃ composite oxide powder (the pH value of thesuspension=8. 2) prepared in Comparative Example No. 2 was used, an N₂gas including 1% CO₂ was distributed for 5 hours. The pH value of asuspension suspending the resulting pretreated CeO₂—ZrO₂—La₂O₃ compositeoxide powder was 6.0.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt(NO₂)₂(NH₃)₂ aqueous solution was2.2.

Example No. 12

The CeO₂—ZrO₂—La₂O₃ composite oxide powder (the pH value of thesuspension=8.2) prepared in Comparative Example No. 2 was used, an N₂gas including 1% CO₂ was distributed for 5 hours. The pH value of asuspension suspending the resulting pretreated CeO₂—ZrO₂—La₂O₃ compositeoxide powder was 6.0.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt(NO₂)₂(NH₃)₂ aqueous solution was3.4.

Comparative Example No. 5

The CeO₂—ZrO₂—La₂O₃ composite oxide powder (the pH value of thesuspension=8.2) prepared in Comparative Example No. 2 was used, wasimmersed in ammonia water whose pH value=10 for 2 hours. It was filteredand washed, was dried at 250° C. for 4 hours, and was thereaftercalcined at 500° C. for2 hours. The pH value of a suspension suspendingthe resulting pretreated CeO₂—ZrO₂—La₂O₃ composite oxide powder was 8.8.

Except that this composite oxide powder was used, Pt was loaded in thesame manner as Example No. 1, thereby making a pelletized catalystsimilarly. The pH value of the used Pt(NO₂)₂(NH₃)₂ aqueous solution was2.2.

TEST AND ASSESSMENT

The resulting respective pelletized catalysts were filled into anassessment apparatus, respectively, and a durability test was carried inwhich they were held at 1,000° C. for 5 hours while alternately flowingan N₂ gas including 2% CO and another N₂ gas including 5% O₂ for every 1minute.

The Pt particle diameter of each of the catalysts after the durabilitytest was measured by a CO pulse adsorption method, and ratios withrespect to the Pt particle diameter of the catalyst of Example No. 5 areset forth in Table 2.

Moreover, each of the catalysts after the durability test was filledinto an assessment apparatus, respectively, the temperature wasincreased from 3° C. to 500° C. while flowing a model gas set forth inTable 1, the CO conversions there between were measured with time. A CO50% conversion temperature (CO50T), a temperature at which the COconversion was 50%, was found from the resulting measured values,respectively, and the results are set forth in Table 2. In addition, therelationship between the pH values of the suspensions and the CO 50%conversion temperatures is illustrated in FIG. 1.

From Table 2, it is understood that the catalyst of each of the exampleswas such that the CO 50% conversion temperature was lower compared withthe catalysts of the comparative examples, and that a high activity wasmaintained even after the durability test. And, since a closecorrelation is appreciable between the CO 50% conversion temperaturesand the Pt particle diameter ratios, it is apparent that maintaining ahigh activity even after the durability test results from the fact thatthe granular growth of Pt was inhibited. Namely, in the catalyst of theexamples, the granular growth of Pt was inhibited during the durabilitytest, as a result, a high purifying activity was revealed even after thedurability test.

And, since each of the examples was different from each of thecomparative examples only in that the pH value of the used suspensionssuspending the composite oxide powders was 7 or less, it is understoodthat the granular growth of Pt is inhibited by using those whosesuspension exhibits a pH value of 7 or less and using noble metal saltaqueous solutions which exhibit a pH value lower than the pH value ofthe suspension. Moreover, from FIG. 1, it is seen that the lower the pHvalue of the suspension was the more the CO purifying activity wasimproved.

Moreover, in Table 2, there is recited the ΔpH, the difference betweenthe pH value of the suspension and the pH value of the Pt salt aqueoussolution, but it is apparent that the smaller ΔpH was the more thegranular growth of Pt was inhibited, and the ΔpH fell in a range of from1 to 5 in the examples.

In addition, it is apparent that it was possible to make the pH value ofthe suspension 7 or less by carrying out a pretreatment, such as theacid treatment, even when the composite oxide whose suspension exhibiteda pH value exceeding 7 was used, and thereby the granular growth of Ptwas inhibited and accordingly a high purifying activity was revealedeven after the durability test. TABLE 1 Gas Species CO₂ O₂ C₃H₆ CO NOH₂O N₂ Concentration 14.1 0.25 0.085 0.12 0.25 2 Balance (%)

TABLE 2 Pt Particle Pt Salt CO50T Dia. pH Δ pH (° C.) Ratio OxideComposition Suspension (% by Weight) pH Ex. #1 CeO₂/ZrO₂ = 50/50 6.8 2.24.6 239 1.6 Ex. #2 CeO₂/ZrO₂/Y₂O₃ = 65/30/5 5.7 2.2 3.5 234 1.5 Ex. #3CeO₂/ZrO₂/La₂O₃ = 60/35/5 5.6 2.2 3.4 229 1.4 Ex. #4 CeO₂/ZrO₂/La₂O₃ =60/35/5 4.8 2.2 2.6 226 1.2 Ex. #5 CeO₂/ZrO₂/La₂O₃ = 60/35/5 4.8 3.4 1.4220 1.0 Ex. #6 CeO₂/ZrO₂/La₂O₃ = 50/45/5 6.0 2.2 3.8 237 1.5 Ex. #7CeO₂/ZrO₂/Nd₂O₃ = 60/35/5 5.9 2.2 3.7 230 1.4 Comp. CeO₂/ZrO₂ = 65/358.8 2.2 6.6 301 4.3 Ex. #1 Comp. CeO₂/ZrO₂/La₂O₃ = 60/35/5 8.2 2.2 6.0262 2.9 Ex. #2 Comp. CeO₂/ZrO₂/La₂O₃ = 60/35/5 8.5 2.2 6.3 267 3.2 Ex.#3 Comp. CeO₂/ZrO₂/La₂O₃ = 60/35/5 8.5 3.4 5.1 258 2.8 Ex. #4 Water-immersion Pretreatment pH Ex. #8 Being Immersed in pH = 2 Nitric 4.4 2.22.2 225 1.4 Acid Aqueous Solution for 2 hours Ex. #9 Being Immersed inpH = 2 Acetic 5.3 2.2 3.1 226 1.4 Acid Aqueous Solution for 2 hours Ex.#10 Being Immersed in pH = 2 4.3 2.2 2.1 236 1.6 Hydrochloric AcidAqueous Solution for 2 hours Ex. #11 Distributing CO₂-contaiing 6.0 2.23.8 224 1.3 Gas for 5 hours Ex. #12 Distributing CO₂-containing 6.0 3.42.6 221 1.1 Gas for 5 hours Comp. Being Immersed in pH = 10 8.8 2.2 6.6278 3.2 Ex. #5 Ammonia Water for 2 hours

1. A catalyst for purifying exhaust gases, the catalyst comprising: anoxide powder having a characteristic that a suspension suspending theoxide powder exhibits a pH value of 7 or less; and a noble metal loadedon the oxide powder by using a noble metal salt solution exhibiting a pHvalue lower than the pH value of the suspension suspending the oxidepowder in pure water.
 2. The catalyst for purifying exhaust gases setforth in claim 1, wherein said oxide powder is a CeO₂-based oxideincluding CeO₂ at least.
 3. The catalyst for purifying exhaust gases setforth in claim 2, wherein said oxide powder includes at least oneelement selected from the group consisting of Zr, La, Y and Nd.
 4. Thecatalyst for purifying exhaust gases set forth in claim 1, wherein saidnoble metal salt solution is a Pt salt aqueous solution.
 5. The catalystfor purifying exhaust gases set forth in claim 1, wherein a difference(ΔpH) between said pH value of the suspension suspending the oxidepowder in pure water and said pH value of the noble metal salt solutionis from 1 to
 5. 6. A process for producing a catalyst for purifyingexhaust gases, the process comprising the steps of: preparing an oxidepowder having a characteristic that a suspension suspending the oxidepowder in pure water exhibits a pH value of 7 or less; and loading anoble metal on the oxide powder by using a noble metal salt solutionexhibiting a pH value lower than the pH value of the suspensionsuspending the oxide powder in pure water.
 7. The process for producinga catalyst for purifying exhaust gases set forth in claim 6, whereinsaid oxide powder is a CeO₂-based oxide including CeO₂ at least.
 8. Theprocess for producing a catalyst for purifying exhaust gases set forthin claim 7, wherein said oxide powder further includes at least oneelement selected from the group consisting of Zr, La, Y and Nd.
 9. Theprocess for producing a catalyst for purifying exhaust gases set forthin claim 6, wherein said noble metal salt solution is a Pt salt aqueoussolution.
 10. The process for producing a catalyst for purifying exhaustgases set forth in claim 6, wherein a difference (ΔpH) between said pHvalue of the suspension suspending the oxide powder in pure water andsaid pH value of the noble metal salt solution is from 1 to 5.